The present invention relates to a fluid control valve device and, more particularly, to a rotary fluid control valve device for controlling distribution of a fluid pressure by means of a relative rotational displacement between a rotor and a sleeve.
In a hydraulic power steering apparatus for reducing a force required to rotate a steering wheel of an automobile, for example, a fluid control valve device is used as a rotary flow path switching valve. This fluid control valve device is used to operate a power cylinder as a moving part in a power steering apparatus in accordance with a steering operation, thereby generating an auxiliary steering force in the steering operation. Examples of such a conventional fluid control valve device are disclosed in Japanese Patent Laid-Open No. 57-178971 and Japanese Utility Model Publication No. 63-30613.
In a flow path switching valve of this type, a rotor provided integrally with an input shaft (stub shaft) coupled to a steering wheel and a sleeve provided integrally with an output shaft (pinion shaft) coupled to wheels to be steered are so assembled as to rotationally displace relative to each other and placed in a valve housing.
A plurality of channels formed in the outer circumferential surface of the rotor and the inner circumferential surface of the sleeve in the direction of circumference are connected to passages communicating with an oil pump as a fluid pressure generating source, an oil tank, and left and right cylinder chambers constituting a power cylinder. Consequently, switching between flow paths of a fluid pressure circuit (hydraulic circuit) can be performed easily and properly, and there is another advantage that the arrangement is simple.
One problem taking place when the above rotary fluid control valve device is used as a flow path switching valve or the like in a power steering apparatus is that in the hydraulic circuit from the pump to the left and right cylinder chambers or the tank through the pressure oil passages in the flow path switching valve, vortex flows or turbulent flows are produced in a flow of pressure oil in the pressure oil passages consisting of the channels and land portions formed in the rotor and the sleeve constituting the control valve and passage holes formed in these portions. This leads to a noise problem that fluid noise, such as so-called hiss noise, is generated due to cavitation.
Such a problem occurs for the reason explained below. That is, the cross section of a flow path defined between the edge of the land portion of the sleeve and a throttle portion (chamfer portion) provided in the rotor, which are throttled gradually and shut off eventually by the relative rotational displacement between the rotor and the sleeve, is decreased in accordance with an amount of rotational displacement. This increases the pressure of a fluid (pressure oil) to be controlled passing through this portion. On the other hand, the flow rate in this portion also increases, and the result is that the fluid (pressure oil) burbles from the circumferential wall of the land portion on the throttle side in accordance with the shape of the throttle portion, thereby generating vortex flows or turbulent flows.
In particular, if a wide or narrow portion or a bent portion is present in the throttle portion to be narrowed or widened in the flow path through which the pressure oil flows, vortex flows or turbulent flows are readily generated, and this results in easy production of fluid noise. Hence, to reduce these portions as few as possible is one technical objective required to be achieved in flow path switching valves of this type.
Of course, various countermeasures have been made conventionally in flow path switching valves of the above sort in order to solve the above noise problem.
For example, Japanese Patent Laid-Open No. 57-160769 or 61-287862 discloses a structure in which a chamfer portion of a rotor, which serves as a throttle portion, is formed as a multistage chamfer portion constituted by at least two acting surfaces with different inclination angles, so that changes in the cross section of a flow path in the direction of a flow, formed by the edge of a land portion of a sleeve and the chamfer portion, are decreased as small as possible.
In this conventional multistage chamfer structure, however, as is apparent from FIG. 6 and FIGS. 7A and 7B, an angle .theta..sub.1 defined between a first acting surface 5a and a second acting surface 5b, which constitute the multistage chamfer portion, and an angle .theta..sub.2 defined between the second acting surface 5b and a tangent l at a point at which the second acting surface 5b and the outer diameter portion of a rotor 1 connect together, are large. Therefore, the fluid burble phenomenon described above cannot be avoided on the downstream side of the throttle portion, and this introduces the fluid noise problem caused by cavitation.
In FIGS. 6, 7A, and 7B, reference numeral 1 denotes a rotor; 2, a channel of the rotor 1; 3, a land portion adjacent to the point at which the second acting surface 5b is connected to the outer diameter portion of the rotor 1, i.e., adjacent to the end of the throttle portion and continuous to the outer diameter portion of the rotor 1; 4, an edge of the rotor 1 formed between the first acting surface 5a adjacent to the channel 2 and the channel 2; 6, a sleeve; 7, a land portion of the sleeve 6; 8, an edge of the land portion 7; and 9, a channel adjacent to the land portion 7.
FIGS. 7A and 7B illustrate the flows of a fluid when the throttle portion described above is throttled during a valve shutoff operation, in which a streamline represented by an arrow A indicates a portion where the flow rate is high, and streamlines represented by arrows B as vortices indicate vortex flows produced by the fluid burble phenomenon.
This phenomenon occurs because the angles .theta..sub.1 and .theta..sub.2 are so large as to exceed a burble limit angle which is generally said to be 5.degree. to 7.degree.. For this reason, taking into account these situations, a demand has arisen for a rotary fluid control valve device of the above sort in which fluid noise generated during throttling is reduced as low as possible.